The phrase "sluggish performance" means that something is moving or working very slowly or with little energy and may not be functioning at its best or expected level. Full definition
I didn't experience any issues with sluggish performance. (ereaderperks.com)
Their earlier devices suffered from sluggish performance and ugly aesthetics. (goodereader.com)
Last year's watch was hampered by sluggish performance and unreliable battery life, which made wearing it more of a chore than a pleasant experience. (theverge.com)