Rakudo needs better performance from Parrot, and Parrot can provide it. Here are several areas of exploration.

Explanation	Difficulty	Amount of Work	Benefit Estimate
FixingPIRVtableOverrides	moderate	low to moderate	5%
FixingConstantSTRINGCaching	moderate	low	big memory savings, 3% performance improvement, further benefits from other string changes (avl_string_cache branch)
Fix freeze/thaw of complex PMCs	difficult	moderate to high	huge startup improvement for Rakudo
Slim parrot_string_t	low	low to moderate	2-3%, depending on workload
Remove PMC struct's sync	low	low	2% at most
Implement sweep-free GC	moderate to high	moderate	10%, depending on workload
Remove PMCProxy	high	moderate	5-7%
Implement garbage-first GC	high	high	10-15%
Immutable Strings	moderate	low (yes, really)	7% (immutable_strings_part1 branch)
Type-specific hashes	low to moderate	low to moderate	3-5%
String equality macro	low	low	2-3%
Store CallSignatures in PBC	moderate	low to moderate	7-8%
Cheaper access to object attributes	moderate to high	moderate	5-7%
Make fast core the default core	low to moderate	low	2-3%
RemoveUnnecessaryDataFromConfig_lib.pbc	low	low to moderate	5-10% Parrot startup (2.926% achieved with r45623, more possibilities await)
Lorito	high	high	orders of magnitude