Being on the insider list doesn't mean that the performance was good enough to get onto the release list.
520 is supposed to be on the "second wave" of W10 Mobile updates, but that doesn't mean it'll get the full Windows 10 Mobile release. Insider user reports I read weren't positive about the low-feature devices: the big problem with updating the 520 isn't the running requirements of the OS, but rather getting enough free storage space on the device to store the OTA updater.
On performance, W10 renders 8.1 app UI faster than 8.1 does, and unlike 8.1., the Windows 10 Store service recompiles the .Net CLI byte-code submitted by developers into native code for download onto the device. Both of these make apps run faster, and there are some API features introduced in 10 (particularly in allowing the use of static binding of control properties) which dramatically improve UI load times if you use them. On the other hand, the W10 OS is doing more in the background than 8.1 was, so overall it's a small performance hit.
Memory is the killer on these devices, though - I've a 530 that I used for developing my recent app, and running on such a low-memory device really taught me to aggressively optimise my heap size.