Hi,
TIFF and PNG imports are usually much faster than ASC import.
It will probably be worth it to convert your data to TIFF or PNG. Instant Terra can export 16-bit TIFF, 16-bit PNG, and 32-bit TIFF.
If you have to handle large terrains, such as 32K x 32K, it may also be interesting to upgrade to Instant Terra Pro. You can send an email to support@wysilab.com to get to information on how to proceed.
Instant Terra Pro offers better performance on large terrains. It also has a cache system to speed up import process (first import is slow, next imports are at least one order of magnitude faster).