How fast is bcrypt_pbkdf anyway?

_first_order_estimation

 The original difficulty parameter for bcrypt is the log2 number of times to rekey the cipher. bhash fixes this number at 64, and instead relies on the PBKDF2 rounds parameter to control difficulty.
 
Default PBKDF2 rounds: 0x10 == 16
=> 20-30 ms for 1 key derivation

-a 100: ~150ms
or about 150ms for -a 100

empirically

user@personal ~ $ time ssh-keygen -o -f test-key -P "" && rm test-key                                                                                                                                     11:01:03
Generating public/private rsa key pair.
Your identification has been saved in test-key.
Your public key has been saved in test-key.pub.
The key fingerprint is:
SHA256:RuKjWtfiGX+O1U31srgL1A91a+IqSdJDSyjDooqd6eE user@personal
The key's randomart image is:
+---[RSA 2048]----+
|                 |
|                 |
|    . . o     . o|
|   . = + o . . oo|
|  . . = S o o.oo.|
| .   . = * ..*oo |
|o..oo + + = o.+  |
|o.++ o = =....   |
| .E   o ooo.o.   |
+----[SHA256]-----+
ssh-keygen -o -f test-key -P ""  0.16s user 0.00s system 99% cpu 0.166 total
user@personal ~ $ time ssh-keygen -o -f test-key -P "iddqd" && rm test-key                                                                                                                                11:01:04
Generating public/private rsa key pair.
Your identification has been saved in test-key.
Your public key has been saved in test-key.pub.
The key fingerprint is:
SHA256:7Z0asvZWbwq/DOOuyY/j2fOn21n7QoOthUYxCfN/KTg user@personal
The key's randomart image is:
+---[RSA 2048]----+
|          o. .   |
|           o+    |
|            .o   |
|         .  o.  .|
|        S .E =...|
|         . .*.*. |
|        . *oo= ..|
|       .oOoO..=o.|
|       oOBBoOBooo|
+----[SHA256]-----+
ssh-keygen -o -f test-key -P "iddqd"  0.25s user 0.00s system 97% cpu 0.257 total
user@personal ~ $ time ssh-keygen -o -f test-key -P "iddqd" -a 100 && rm test-key                                                                                                                         11:01:14
Generating public/private rsa key pair.
Your identification has been saved in test-key.
Your public key has been saved in test-key.pub.
The key fingerprint is:
SHA256:ngRngk5lPo8AfsryGJ++Nr0TSE8MVsQFbLH2AC3+x+0 user@personal
The key's randomart image is:
+---[RSA 2048]----+
|  o*=++          |
| .+o=*           |
| o.=O = o        |
| .o*o+ O         |
|o.o=..o.S        |
| *..+ oo..       |
|. +. o .o        |
| .o o   E        |
| .oo.o           |
+----[SHA256]-----+
ssh-keygen -o -f test-key -P "iddqd" -a 100  0.88s user 0.00s system 99% cpu 0.883 total