Elasticsearch random slow queries troubleshooting - performance

Question

I moved my data from a Elasticsearch cluster to another with powerful hardware
(4 nodes / 2CPU 8GB RAM each one with 4GB on JVM per machine // old cluster had 3 nodes with 1 cpu each one and 2GB on JVM per machine)
but I am randomly experiencing some very slow query responses that I didn't have on the old cluster.
Same ES version on both nodes 6.8.14 and same document numbers / shards
(62GB data / 106 million documents).
Launching the queries from kibana using the profiler on the new cluster
is showing that most of the time is spent in this phase :
"collector" : [
{
"name" : "CancellableCollector",
"reason" : "search_cancelled",
"time_in_nanos" : 5777066437,
"children" : [
{
"name" : "MultiCollector",
"reason" : "search_multi",
"time_in_nanos" : 5756203807,
"children" : [
{
"name" : "SimpleTopScoreDocCollector",
"reason" : "search_top_hits",
"time_in_nanos" : 747674917
},
{
"name" : "MultiBucketCollector: [[min_price, max_price, in_stock, out_of_stock, category, agg_h, mdata, agg_att, agg_f]]",
"reason" : "aggregation",
"time_in_nanos" : 4966026553
}
]
}
]
}
]
CPU usage per node is pretty low (5/10%) and load average is OK (0.50)
Launching the same query again in short time lowers the response from 8 sec to 0.4 sec (due to ES caching I guess), also removing the "aggs" part from the query seems to fix the issue so poor performance is actually during the aggregators phase.
Anyway I do not understand why this slowdown occurs only on the new "better" cluster... how can I optimize the performance or better troubleshoot ?