diff --git a/library/jpeg_xl_util.py b/library/jpeg_xl_util.py
new file mode 100644
index 00000000..c2e3a393
--- /dev/null
+++ b/library/jpeg_xl_util.py
@@ -0,0 +1,186 @@
+# Modified from https://github.com/Fraetor/jxl_decode Original license: MIT
+# Added partial read support for up to 200x speedup
+
+import os
+from typing import List, Tuple
+
+class JXLBitstream:
+    """
+    A stream of bits with methods for easy handling.
+    """
+
+    def __init__(self, file, offset: int = 0, offsets: List[List[int]] = None):
+        self.shift = 0
+        self.bitstream = bytearray()
+        self.file = file
+        self.offset = offset
+        self.offsets = offsets
+        if self.offsets:
+            self.offset = self.offsets[0][1]
+            self.previous_data_len = 0
+            self.index = 0
+        self.file.seek(self.offset)
+
+    def get_bits(self, length: int = 1) -> int:
+        if self.offsets and self.shift + length > self.previous_data_len + self.offsets[self.index][2]:
+            self.partial_to_read_length = length
+            if self.shift < self.previous_data_len + self.offsets[self.index][2]:
+                self.partial_read(0, length)
+            self.bitstream.extend(self.file.read(self.partial_to_read_length))
+        else:
+            self.bitstream.extend(self.file.read(length))
+        bitmask = 2**length - 1
+        bits = (int.from_bytes(self.bitstream, "little") >> self.shift) & bitmask
+        self.shift += length
+        return bits
+
+    def partial_read(self, current_length: int, length: int) -> None:
+        self.previous_data_len += self.offsets[self.index][2]
+        to_read_length = self.previous_data_len - (self.shift + current_length)
+        self.bitstream.extend(self.file.read(to_read_length))
+        current_length += to_read_length
+        self.partial_to_read_length -= to_read_length
+        self.index += 1
+        self.file.seek(self.offsets[self.index][1])
+        if self.shift + length > self.previous_data_len + self.offsets[self.index][2]:
+            self.partial_read(current_length, length)
+
+
+def decode_codestream(file, offset: int = 0, offsets: List[List[int]] = None) -> Tuple[int,int]:
+    """
+    Decodes the actual codestream.
+    JXL codestream specification: http://www-internal/2022/18181-1
+    """
+
+    # Convert codestream to int within an object to get some handy methods.
+    codestream = JXLBitstream(file, offset=offset, offsets=offsets)
+
+    # Skip signature
+    codestream.get_bits(16)
+
+    # SizeHeader
+    div8 = codestream.get_bits(1)
+    if div8:
+        height = 8 * (1 + codestream.get_bits(5))
+    else:
+        distribution = codestream.get_bits(2)
+        match distribution:
+            case 0:
+                height = 1 + codestream.get_bits(9)
+            case 1:
+                height = 1 + codestream.get_bits(13)
+            case 2:
+                height = 1 + codestream.get_bits(18)
+            case 3:
+                height = 1 + codestream.get_bits(30)
+    ratio = codestream.get_bits(3)
+    if div8 and not ratio:
+        width = 8 * (1 + codestream.get_bits(5))
+    elif not ratio:
+        distribution = codestream.get_bits(2)
+        match distribution:
+            case 0:
+                width = 1 + codestream.get_bits(9)
+            case 1:
+                width = 1 + codestream.get_bits(13)
+            case 2:
+                width = 1 + codestream.get_bits(18)
+            case 3:
+                width = 1 + codestream.get_bits(30)
+    else:
+        match ratio:
+            case 1:
+                width = height
+            case 2:
+                width = (height * 12) // 10
+            case 3:
+                width = (height * 4) // 3
+            case 4:
+                width = (height * 3) // 2
+            case 5:
+                width = (height * 16) // 9
+            case 6:
+                width = (height * 5) // 4
+            case 7:
+                width = (height * 2) // 1
+    return width, height
+
+
+def decode_container(file) -> Tuple[int,int]:
+    """
+    Parses the ISOBMFF container, extracts the codestream, and decodes it.
+    JXL container specification: http://www-internal/2022/18181-2
+    """
+
+    def parse_box(file, file_start: int) -> dict:
+        file.seek(file_start)
+        LBox = int.from_bytes(file.read(4), "big")
+        XLBox = None
+        if 1 < LBox <= 8:
+            raise ValueError(f"Invalid LBox at byte {file_start}.")
+        if LBox == 1:
+            file.seek(file_start + 8)
+            XLBox = int.from_bytes(file.read(8), "big")
+            if XLBox <= 16:
+                raise ValueError(f"Invalid XLBox at byte {file_start}.")
+        if XLBox:
+            header_length = 16
+            box_length = XLBox
+        else:
+            header_length = 8
+            if LBox == 0:
+                box_length = os.fstat(file.fileno()).st_size - file_start
+            else:
+                box_length = LBox
+        file.seek(file_start + 4)
+        box_type = file.read(4)
+        file.seek(file_start)
+        return {
+            "length": box_length,
+            "type": box_type,
+            "offset": header_length,
+        }
+
+    file.seek(0)
+    # Reject files missing required boxes. These two boxes are required to be at
+    # the start and contain no values, so we can manually check there presence.
+    # Signature box. (Redundant as has already been checked.)
+    if file.read(12) != bytes.fromhex("0000000C 4A584C20 0D0A870A"):
+        raise ValueError("Invalid signature box.")
+    # File Type box.
+    if file.read(20) != bytes.fromhex(
+        "00000014 66747970 6A786C20 00000000 6A786C20"
+    ):
+        raise ValueError("Invalid file type box.")
+
+    offset = 0
+    offsets = []
+    data_offset_not_found = True
+    container_pointer = 32
+    file_size = os.fstat(file.fileno()).st_size
+    while data_offset_not_found:
+        box = parse_box(file, container_pointer)
+        match box["type"]:
+            case b"jxlc":
+                offset = container_pointer + box["offset"]
+                data_offset_not_found = False
+            case b"jxlp":
+                file.seek(container_pointer + box["offset"])
+                index = int.from_bytes(file.read(4), "big")
+                offsets.append([index, container_pointer + box["offset"] + 4, box["length"] - box["offset"] - 4])
+        container_pointer += box["length"]
+        if container_pointer >= file_size:
+            data_offset_not_found = False
+
+    if offsets:
+        offsets.sort(key=lambda i: i[0])
+    file.seek(0)
+
+    return decode_codestream(file, offset=offset, offsets=offsets)
+
+
+def get_jxl_size(path: str) -> Tuple[int,int]:
+    with open(path, "rb") as file:
+        if file.read(2) == bytes.fromhex("FF0A"):
+            return decode_codestream(file)
+        return decode_container(file)
diff --git a/library/train_util.py b/library/train_util.py
index 1ed1d3c2..53799081 100644
--- a/library/train_util.py
+++ b/library/train_util.py
@@ -113,14 +113,16 @@ except:
 # JPEG-XL on Linux
 try:
     from jxlpy import JXLImagePlugin
+    from library.jpeg_xl_util import get_jxl_size
 
     IMAGE_EXTENSIONS.extend([".jxl", ".JXL"])
 except:
     pass
 
-# JPEG-XL on Windows
+# JPEG-XL on Linux and Windows
 try:
     import pillow_jxl
+    from library.jpeg_xl_util import get_jxl_size
 
     IMAGE_EXTENSIONS.extend([".jxl", ".JXL"])
 except:
@@ -1463,6 +1465,8 @@ class BaseDataset(torch.utils.data.Dataset):
                 )
 
     def get_image_size(self, image_path):
+        if image_path.endswith(".jxl") or image_path.endswith(".JXL"):
+            return get_jxl_size(image_path)
         # return imagesize.get(image_path)
         image_size = imagesize.get(image_path)
         if image_size[0] <= 0: