Speed up export_grdecl for ascii field

[jonathan-eq]

Issue
Resolves #12658

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Add backport label to latest release (format: 'backport release-branch-name')

[jonathan-eq]

A case based on test_field_parameter_persistence_to_grdecl had these results (esmda create_runpath_duration):

FieldParameters	Old	New
100_000	X	y
500_000	6.2315s	4.239300753964926s
1_000_000	50.0562s	34.4127s
5_000_000	x	y