Here is a first beta test for Teensyduino 1.49.

USB optimization is the major change in this beta. I hope you'll have a moment to give the new optimized USB serial a try. I will soon start using this new optimized USB code as the basis for the many other USB types...

EDIT: links removed, please use 1.49-beta3

Changes since Teensyduino 1.48

Optimize USB serial on Teensy 4.0
Move USB serial buffers from ITCM to OCRAM on Teensy 4.0
Fix stalled USB serial transmit if multiple of 64 bytes on Teensy 4.0
Support different USB descriptors for 12 vs 480 speed on Teensy 4.0
Add FLASHMEM keyword
Arduino memory usage for Teensy 4.0 shows only RAM1 (512K) bank usage
Fix Audio lib delay effect maximum (2.4 sec) on Teensy 4.0
Fix missing ITCM padding from Teensy 4.0 memory usage
Fix empty code on Teensy 3.x if yield() overridden and no core lib functions used
Reduce DTCM memory usage by USB descriptors on Teensy 4.0
Fix USB transfer complete callbacks on Teensy 4.0
Fix Audio lib memory limit on Teensy 4.0
WS2812Serial add functions for better compatibility with Adafruit_NeoPixel
Windows teensy_serialmon (hopefully) doesn't lock up (but still too much buffering!)