I’m glad the technique worked for you and it sounds great that you can mostly take advantage of the long strings as variables.
There is no rate-limiting mechanism built in to Particle variables that I know of, but I’m not surprised that there are practical limits to how much back to back communication you can do. Don’t forget that every time your HTML reads a Particle variable, it is going from browser to Particle server to device and back. I would think that the last link of that chain, the device itself, is the rate determining step since the cloud will only issue one request at a time, so any work you can do to improve the “loop time” on your device would likely improve things.
Another limiting factor is how long it takes your device to get message to/from the Particle cloud. I did some experiments recording times in the distant past and found that my devices (in the Boston area in the USA) see about 140ms of latency for each message. I did these measurements before the Particle cloud was scaled up to the extent it is now, so the exact number is likely different today, but the idea is that if it takes about 140ms up to the cloud and the same back down, you cannot do back-to-back transactions any faster that about 280ms or about 3.6 per second. I’m sure these numbers are not correct for the today’s cloud, but doing similar measurements at your location might give you confidence in your delay values.