Just like Linux or Windows or OS X or UNIX you load once and keep using programs mostly as many as you can fit into memory without having to load more from disk.
This is the concept behind my RXB 32K swaps using SAMS.
LOAD ONCE and use as many programs only in memory (SAMS) without using disk as little as possible.
But as Tursi and others have pointed out TI99/4A load times are limited by using VDP as a buffer so slows load times by depressing numbers.
TIPI may be like Tursi indicated use PAB CPU RAM buffer at >4000 which would be considerably faster then using VDP as a PAB buffer.