Does this limit transfer speeds or pcie lanes?

ThunderBolt-2 at 20,000 Mega bits/sec with talking numbers of 10 bits per Byte including overhead yields a theoretical maximum of 2,000 Mega Bytes/sec.

Your seeing actual 1300 read is about what I would expect. The drives have some latency.

Intel ThunderBolt-3 Macs can get up to nominal 2500 M Bytes/sec on an appropriately fast SSD drive in a ThunderBolt-3 enclosure, provided they use a non-busy ThunderBolt controller on the Mac (not shared controller busy servicing displays or other drives on a second Mac port) supported by a typical 32 G bits/sec (PCIe2 x4, or PCIe3 x2 amount of bandwidth).

That is because Thunderbolt 2 is half as fast as Thunderbolt 3

Real world speed results with raided drives (esp non SSD ones) are often quite a bit below the theoretical speeds that the carrier (in this case TB2) can offer.

I see different results also depending on the type (size and number) of files being accessed. EG a large video file being streamed will pick up speed and “flow” nicely. Whereas accessing multiple files at once (EG a music sample library playing a keyboard) will place a very high demand on seek times.

I experienced also a disappointing result with an iMac Pro until I learned that, despite using external SSD’s, attaching a couple of TB monitors to the TB busses of the iMac Pro basically throttled the whole TB system - so a busy TB buss will struggle more than a dedicated buss.

It is only 1500MBps. 2500MBps maybe possible using Thunderbolt 3 and the PC RAID Software.

https://sabrent.com/products/ec-t3dn