The best answer can be done by the one that own both models, but excepting those that get a old test v.1 of the 1000X-HD I don't think that someone will buy both models for the future.
This models does not have a CPU in terms of clasic arhitecture, but if you are refering to the application processor, 1000X-HD has a more recent and advanced one - Zynq 7000 UltraScale+ MPSoC, while 2000X-HD has an older version of Zinq - Zynq 7000 SoC.
But ... while 1000X-HD is doing the signal processing with the FPGA logic included in the same Zinq MPSOC, 2000X-HD has 2 other more powerfull FPGA's available: Spartan 7 and Artix 7 to do the processing nedeed for 2x2 Gsa/s and 4x12 bit channels at 500 MHz BW.
So the difference in power procesing of the signal became obvious.
During use SDS100-X-HD is not slow at all, except when you have more math function simultaneously for ex. FFT or nested functions.
So in terms of usability 1000X-HD does not show slowness as an obvious issue, only if you look for it you can find situations when it can be revealed. But I didn't find scopes that can't be kneel if you really want.
P.S. If you look for a model with a clasic CPU from Siglent they have SDS7000A wich feature an x86 architecture.