SS IPS, Rapid IPS, Fast IPS, Agile Splendor IPS, etc. are basically marketing terms by monitor manufacturers.
Just like how AU Optronics’ AHVA, Innolux’s AAS and BOE’s ADS are all IPS-type panels, Rapid IPS, Fast IPS, etc. are also the same family of IPS panels and share common traits, such as wide viewing angles and consistent colors.
As IPS panels got faster over time, some manufacturers started giving different names to the panels their monitor is using – therefore, it’s mostly just for marketing purposes.
MSI calls their monitors with IPS panels that have a fast pixel response time speed “Rapid IPS”, while Acer calls them “Agile-Splendor IPS” – even if they’re using the same IPS (AHVA) panel made by AU Optronics, for instance.
Some manufacturers still choose to simply call their fast IPS panels “IPS.” A Rapid IPS panel might not necessarily be faster than an Agile-Splendor IPS panel or even an “IPS” panel.
Even if two monitors use the exact same panel, one might be faster than the other due to better pixel response time overdrive optimization. Monitors with the same panel can also use different backlights, resulting in different color gamut, brightness and contrast performance.
Of course, every panel has its native performance. For example, the AUO M270DAN08.D panel might be faster than the Innolux M270KCJ-K7E panel and have a higher contrast ratio than the LM270WQA Nano IPS panel.
In the end, if you want an IPS monitor with a fast response time speed, you’ll need to check exactly which panel it’s using and how well its response time overdrive is optimized instead of relying on marketing terms.