While the specific reason for SL_DEFAULT_FALLBACK_RETRIES = 20000 is not known, workloads that use switchless calls require fine-performance tuning. The Intel Software Guard Extensions (Intel SGX)  Developer Reference for Linux* states:
Switchless Calls is an advanced feature. It requires additional worker threads and configuration, performance testing, and tuning. It should be used for workloads that require fine performance tuning. Misconfiguration may result in underutilized worker threads, which consumes CPU time while not serving any tasks.