First air qualiry if Delhi is poor and so there was smog in IND SL match in betwee innings which was causing bit of problems for players, specially fast bowlers
Sri Lankan players who clearly were seeing scoreline of India going past 500 yet seeing no sign of declaration took advantage of it, they put on air masks and then started acting like they can't even breath and stuff to force declaration, and then when team india refused to declare, 2 of SL bowlers went off field saying difficulty in playing and then india had no choice but to declare
Suddenly after declaration, SL players were seen laughing as if nothing happened and they came to bat without any masks and other players were chilling in dressing room
So while air quality was kind of pooe, SL players were also acting to take advantage of situation
and one more factor
SL players comes from place where pollution level is much lower then Delhi so it's likwly they were facing more problems then Indian players who are used to these conditions but then again umpires who also comes from other countries were seemingly fine so that's that too.