There's so many factors that it could be, that its hard to say what the cause is. The Ubiquiti access point you tried is a 802.11ac Wave 2 4x4 device, whereas the MR32 is a 802.11ac Wave 1 2x2 device, so it is possible this is purely a bandwidth issue, and based on that you need to upgrade the MR32 to take advantage of more up to date technology. Likewise, the MR34 is a 3x3 device, so will give greater throughput, but being on a different site its hard to compare directly as there may well be other factors at play, e.g. density, construction/walls, interference.